Species Profile – Brittle Stars
Brittle stars are aptly named — when specimens are brought into the lab for us to photograph they are generally missing arms. We always get blamed, but our response always is, “they came to us that way, honest.” Seems their arms tend to fall off when touched by a scientist or grabbed by a predator.
Brittle stars, or ophiuroids (ophis means snake in Greek), are echinoderms and are closely related to starfish. They crawl across the seafloor or in coral or rubble using their flexible arms for locomotion. The movement of their arms is indeed snake-like. They generally have five long, slender, whip-like arms, which may reach up to 24 inches in length on the largest specimens.
Luckily for them, in spite of being fragile, ophiuroids can readily regenerate lost arms or arm segments. They use this ability to cast off an arm in a way similar to how some lizards deliberately shed part of their tails to escape and confuse predators. Some brittle stars are also able to emit a green light when disturbed. Each arm is supported by a central internal skeletal support (ossicle). Like starfish, brittle stars have tube feet, but those of brittle stars lack suckers. The tube feet help more with feeding than with locomotion.
The central disk of ophiuroids contains all of the internal organs of digestion and reproduction. So, unlike starfish, these organs are never found in the arms of brittle stars. Look closely at the underside of the disk. The star shape is from the five moveable jaw segments surrounding the mouth. Ophiuroids are generally scavengers or detritivores. Small organic particles are moved into the mouth by the tube feet. They may also prey on small crustaceans or worms. The paired sacs between each arm are called bursae. These are the primary sites for gas exchange and excretion in brittle stars. In some species, the bursae are also used as brooding chambers for developing larvae.